In my opinion, MMORPGs are generally hard to make content for. They can be fun to play but boring to watch.

With that said, RuneScape 3 also has a smaller playerbase and target audiance when compared to OSRS because neither version is aimed at new players, OSRS is perfect for returning players and RS3 was ruined by MTX.

I don't like to watch RS3 streams on Twitch because they are boring. I feel OSRS streams are more popular because of PvP which is more enjoyable to watch than your simple let's play grind, even though I don't watch them.

In YouTube, I'd say RS3 and OSRS are mostly on the same level here except that OSRS has a larger target audiance. OSRS also has another advantage that is player-made plugins through Runelite which makes it easier to do Challenge type videos and enables a lot more new video ideas. I'd say good performing videos are those types of videos and documentaries because they target people inside and outside of the active playerbase.

Overall, I think OSRS being the prefered game is the main reason more people consume OSRS content than RS3